The invention relates to a method of measuring the damping capability of a test part, e.g. a brake rotor. The invention is useful for predicting the noise level (or squeal) produced by a brake rotor during a vehicle braking operation.
U.S. Pat. No. 6,014,899 issued Jan. 18, 2000, relates to a method of measuring the vibration damping capability of a part, such as a brake rotor, wherein a vibrational input of a specified frequency is applied at one point on the part surface, and a vibrational output is measured at a point located directly across from the input point, i.e. one hundred eighty degrees from the input point. The amplitude of the vibrational wave at the output location is measured while the vibrational input is terminated abruptly, so that the output vibration is allowed to decay. During the decay period the amplitude of the wave motion gradually decreases from the amplitude measured at the instant the input wave is terminated. The rate of decay of the output wave provides an indication of the vibration damping capability of the tested part. The slope of the output wave amplitude peaks during the decay period can be used to compute a damping capability known as the Q-factor.
It is noted in U.S. Pat. No. 6,014,899 that the Q-factor for a given test part can vary, depending on the point where the input vibrational force is applied (measured circumferentially around the part surface). Variability of the measured Q-factor is due in part to the previously unaccounted presence of a second or twin vibrational bending mode displaced forty five degrees away from the first, as measured circumferentially around the test part. The frequency of the second vibrational mode can vary from the applied frequency as much as five hertz. When the measured Q-factor is plotted against the vibrational force input location on an X-Y graph, the Q-factor varies sinusoidally, with a period of forty five degrees. The point of maximum vibrational excitation (and maximum Q-factor) is termed the antinode, and the point of minimum vibrational excitation (and minimum Q-factor) is termed the node.
In order to compensate for variations in the measured Q-factor (due to variation in the location where the measurement is taken), it is proposed in U.S. Pat. No. 6,014,899 to take measurements at various points along the part circumference. The Q factor measurements can be averaged to provide a single Q-factor representative of the damping capability of the tested part.
U.S. patent application Ser. No. 09/519,485, filed on Mar. 6, 2000 relates to an improvement on the invention disclosed in U.S. Pat. No. 6,014,899. U.S. Ser. No. 09/519,485 provides a method for locating the antinodes on the test part, so that the Q-factor needs to be computed only for one antinode location for each of the twin nodes and points proximate to that antinode location, e.g. locations within about nine degrees on either side of the located antinode.
U.S. patent application Ser. No. 09/578,341 (attorney docket No. 99-1625), filed on May 24, 2000, now U.S. Patent No. 6,257,063, relates to a further improvement on the invention disclosed in U.S. Pat. No. 6,014,899. The referenced patent application provides a method of vibrating the test part with an exciter coil that is supplied with an AC current whose amplitude is symmetrical with respect to the zero current axis. With such an arrangement the vibration frequency of the tested part is twice the current excitation frequency. The method used to vibrate the part is advantageous in that a relatively low cost amplifier can be used, while achieving an A.C. waveform that is repeatable, without uncertainties as to amplifier performance.
A common feature of the inventions disclosed in U.S. Pat. No. 6,014,899 and the referenced patent applications is the use of a decaying output wave motion to measure the damping capability of the test part. In each case, changes in amplitude of the decaying wave are measured. For example, in an illustrative arrangement a real time analyzer measures the time required for the decaying wave to experience a predetermined change in amplitude, e.g. from a 90 decibel value to a 65 decibel value. Decay measurements are taken in real time at a specified invariant wave frequency.
The present invention relates to a method of measuring the vibration damping capability of a test part, wherein the tested part is subjected to various different steady state frequencies in order to determine a damping capability Q-factor. As compared to prior methods carried out in the time domain, the present invention is carried out in the frequency domain.
In general, when the test part is subjected to the resonant frequency the amplitude of the output wave will be relatively high. As the frequency of the applied wave is increased or decreased from the resonant frequency, the amplitude of the output (measured) wave will decrease from its maximum. It is possible to construct an X-Y graph plotting the applied frequency against the amplitude of the output wave. The plotted curve will have a shape that is generally sinusoidal, with the resonant frequency being located at the peak of the sinusoid. Preferably the raw data points are passed through a fast Fourier transform analyzer, to improve the accuracy and repeatability of the final test result.
It is believed that by subjecting the test part to a range of different frequency input signals, the computed Q-factor will be a reliable representation of the damping capability of the test part. The computed Q-factor provides an indication of damping performances that might be expected under a variety of different operating conditions. The measurements used in practice of this invention can be performed quickly with the aid of a computer. Computation of a single Q-factor can be accomplished in a relatively short time period.
Specific features of the invention will be apparent from the attached drawings and description of an apparatus that can be used in practice of the invention.